1. Field of the Invention
Embodiments disclosed herein relate generally to composite materials used in cutting tools.
2. Background Art
Historically, there have been two types of drill bits used drilling earth formations, drag bits and roller cone bits. Roller cone bits include one or more roller cones rotatably mounted to the bit body. These roller cones have a plurality of cutting elements attached thereto that crush, gouge, and scrape rock at the bottom of a hole being drilled. Several types of roller cone drill bits are available for drilling wellbores through earth formations, including insert bits (e.g. tungsten carbide insert bit, TCI) and “milled tooth” bits. The bit bodies and roller cones of roller cone bits are conventionally made of steel. In a milled tooth bit, the cutting elements or teeth are steel and conventionally integrally formed with the cone. In an insert or TCI bit, the cutting elements or inserts are conventionally formed from tungsten carbide, and may optionally include a diamond enhanced tip thereon.
The term “drag bits” refers to those rotary drill bits with no moving elements. Drag bits are often used to drill a variety of rock formations. Drag bits include those having cutting elements or cutters attached to the bit body, which may be a steel bit body or a matrix bit body formed from a matrix material such as tungsten carbide surrounded by an binder material. The cutters may be formed having a substrate or support stud made of carbide, for example tungsten carbide, and an ultra hard cutting surface layer or “table” made of a polycrystalline diamond material or a polycrystalline boron nitride material deposited onto or otherwise bonded to the substrate at an interface surface.
Thus, some of the primary materials used in the formation of various components in drill bits, as well as other cutting tools, include tungsten carbide and diamond particles. In composites formed with tungsten carbide and diamond particles, the resulting composite includes the hard particle surrounded by metal binder, typically cobalt, which acts as a matrix. The individual hard particles thus are embedded in a matrix of a relatively ductile metal such that the ductile metal matrix provides the necessary toughness, while the grains of hard material in the matrix furnish the necessary wear resistance. The ductile metal matrix also reduces crack formation and suppresses crack propagation through the composite material once a crack has been initiated.
Polycrystalline diamond (PCD), a composite material formed from diamond particles, comprises a polycrystalline mass of diamonds (typically synthetic) that are bonded together to form an integral, tough, high-strength mass or lattice. A metal catalyst, such as cobalt, may be used to promote recrystallization of the diamond particles and formation of the lattice structure. Thus, cobalt particles are typically found within the interstitial spaces in the diamond lattice structure. The resulting PCD structure produces enhanced properties of wear resistance and hardness, making PCD materials extremely useful in aggressive wear and cutting applications where high levels of wear resistance and hardness are desired. However, cobalt has a significantly different coefficient of thermal expansion as compared to diamond. Therefore, upon heating of a diamond table, the cobalt and the diamond lattice will expand at different rates, causing cracks to form in the lattice structure and resulting in deterioration of the diamond table.
In order to obviate this problem, strong acids may be used to “leach” the cobalt from the diamond lattice structure. Examples of “leaching” processes can be found, for example in U.S. Pat. Nos. 4,288,248 and 4,104,344. Briefly, a hot strong acid, e.g., nitric acid, hydrofluoric acid, hydrochloric acid, or perchloric acid, or combinations of several strong acids may be used to treat the diamond table, removing at least a portion of the catalyst from the PDC layer.
Cemented tungsten carbide composites, such as WC—Co, are well known for their mechanical properties of hardness, toughness and wear resistance, making the composites a popular material of choice for use in such industrial applications as mining and drilling where their mechanical properties are highly desired. Because of the desired properties, cemented tungsten carbide has been the dominant material used as cutting tools for machining, hard facing, wear inserts, and cutting inserts in rotary cone rock bits, and substrate bodies for drag bit shear cutters. The mechanical properties associated with cemented tungsten carbide and other cermets, especially the unique combination of hardness toughness and wear resistance, make these materials more desirable than either metals or ceramics alone.
Many factors affect the durability of a tungsten carbide composite in a particular application. These factors include the chemical composition and physical structure (size and shape) of the carbides, the chemical composition and microstructure of the matrix metal or alloy, and the relative proportions of the carbide materials to one another and to the matrix metal or alloy. Generally, as the tungsten carbide particle size and/or cobalt content decrease, higher hardness, compressive strength, and wear resistance, but lower toughness is achieved. Conversely, larger particle sizes and/or higher cobalt content yields high toughness and impact strength, but lower hardness.
Many different types of tungsten carbides are known based on their different chemical compositions and physical structure. Among the various types of tungsten carbide commonly used in drill bit components are cast tungsten carbide, macro-crystalline tungsten carbide, carburized tungsten carbide, and cemented tungsten carbide (also known as sintered tungsten carbide).
One type of tungsten carbide is macro-crystalline carbide. This material is essentially stoichiometric tungsten carbide created by a thermite process. Most of the macro-crystalline tungsten carbide is in the form of single crystals, but some bicrystals of tungsten carbide may also form in larger particles. Single crystal stoichiometric tungsten carbide is commercially available from Kennametal, Inc., Fallon, Nev.
Carburized carbide is yet another type of tungsten carbide. Carburized tungsten carbide is a product of the solid-state diffusion of carbon into tungsten metal at high temperatures in a protective atmosphere. Sometimes, it is referred to as fully carburized tungsten carbide. Such carburized tungsten carbide grains usually are multi-crystalline, i.e., they are composed of tungsten carbide agglomerates. The agglomerates form grains that are larger than the individual tungsten carbide crystals. These large grains make it possible for a metal infiltrant or an infiltration binder to infiltrate a powder of such large grains. On the other hand, fine grain powders, e.g., grains less than 5 μm, do not infiltrate satisfactorily. Typical carburized tungsten carbide contains a minimum of 99.8% by weight of tungsten carbide, with a total carbon content in the range of about 6.08% to about 6.18% by weight.
Cast tungsten carbide, on the other hand, is formed by melting tungsten metal (W) and tungsten monocarbide (WC) together such that a eutectic composition of WC and W2C, or a continuous range of compositions therebetween, is formed. Cast tungsten carbide typically is frozen from the molten state and comminuted to a desired particle size.
A fourth type of tungsten carbide, which has been typically used in hardfacing, is cemented tungsten carbide, also known as sintered tungsten carbide. Sintered tungsten carbide comprises small particles of tungsten carbide (e.g., 1 to 15 microns) bonded together with cobalt. Sintered tungsten carbide is made by mixing organic wax, tungsten carbide and cobalt powders, pressing the mixed powders to form a green compact, and “sintering” the composite at temperatures near the melting point of cobalt. The resulting dense sintered carbide can then be crushed and comminuted to form particles of sintered tungsten carbide.
For conventional cemented tungsten carbide, the mechanical property of fracture toughness is inversely proportional to hardness, and wear resistance is proportional to hardness. Although the fracture toughness of cemented tungsten carbide has been somewhat improved over the years, it is still a limiting factor in demanding industrial applications such as high penetration drilling, where cemented tungsten carbide inserts often exhibit gross brittle fracture that can lead to catastrophic failure. Traditional metallurgical methods for enhancing fracture toughness, such as grain size refinement, cobalt content optimization, and strengthening agents, have been substantially exhausted with respect to conventional cemented tungsten carbide.
PCD, discussed above, is another type of material that is known to have desirable properties of hardness, and wear resistance, making it especially suitable for those demanding applications described above where high wear resistance is desired. However, this material also suffers from the same problem as cemented tungsten carbide, in that it also displays properties of low fracture toughness that can result in gross brittle failure during usage.
Regardless of the type of material used in a particular drilling application, designers continue to seek improved properties (such as improved wear resistance, thermal resistance, fracture toughness etc.) in the composite material. Unfortunately, further compounding the low fracture toughness is that, in the formation of the various types of composite materials, conventional powder mixing techniques and infiltration methods do not always uniformly distribute the diamond or tungsten carbide particles in the binder material. Thus, non-homogenous microstructures frequently result, where the hard material (i.e., diamond or tungsten carbide) agglomerates and the binder metal pools. Because the binder material, which provides ductility and thus the fracture toughness to a composite material, is not homogenously distributed through the composite material, the composite material possesses a lower fracture toughness and impact strength. Further, in the particular context of diamond composites, low thermal stability arises from the difference in thermal expansion coefficients.
One suggested technique, both in the context of hardfacing and inserts, has been to apply a “coating” layer around the hard particles, either diamond or tungsten carbide particles, to improve various mechanical properties, including thermal stability and toughness. Such disclosure of coating hard particles is described, for example, in U.S. Pat. Nos. 5,755,299, 6,102,140, 6,106,957, and 6,138,779.
In U.S. Pat. No. 6,106,957, it is disclosed that by coating the super-hard particles in the matrix material, the resulting microstructure may be obtained where the resulting composite material is generally substantially free of large clusters of grains of the super-hard material and the average size of the matrix metal pools in the composite is smaller than the average size of the grains of the super-hard material.
Currently practiced commercial methods of coating include wet chemistry, physical vapor deposition (PVD), chemical vapor deposition (CVD), and plasma-enhanced CVD (PE-CVD). While these methods offer outstanding coating processes for flat substrates and large particles where relatively thick and non-uniform coatings are acceptable, they do not allow for the controlled ultra-thin coating of individual ultra-fine particles such as those used in the formation of composite materials for cutting tools. Application of thick, non-uniform coatings may result in alteration of the bulk properties of the particulate material, and thus material properties of the resulting composite material.
Accordingly, there exists a continuing need for improvements in the material and/or thermal properties of composite materials used drilling applications.